1. Field of the Invention
The present invention relates to an imaging lens which forms on an imaging element such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor an image of an object, and relates to a small-sized imaging lens to be mounted on a PDA (Personal Digital Assistant) such as a cellular phone and the like.
2. Description of the Related Art
In recent years, camera functions are mounted on almost all cellular phones. Recently, cellular phones with high-resolution camera function comparable with a digital still camera mounted thereon have been introduced. Imaging devices realizing such camera function are being downsized further in accordance with increasing demands for downsizing and slimming cellular phones. Inevitably, imaging lenses fixed to the imaging devices are also subjected to intense demands for downsizing. Further, imaging lenses realizing high optical performance capable of being applied to increasing pixels and increasing resolution of the imaging element are being demanded intensely.
Accompanying the downsizing and increasing pixels of the imaging element, pixel size is miniaturized further and the density thereof is increased. Recently, imaging elements with a pixel pitch of less than 1.4 micron are proposed. As the performance required for the imaging lens corresponding to such imaging element, the simple accomplishment of small aberrations is not enough. This is because bright optical system having sufficient resolution, that is, imaging lens with larger aperture ratio, is demanded intensely. Conventionally, imaging lens having a three-lens configuration has been widely proposed. However, imaging lens having four-lens configuration and five-lens configuration are also being proposed, so as to be adopted in above-described imaging elements.
For example, the imaging lens according to Japanese Patent Application Laid-Open No. 2007-219079 (Patent document 1) comprises, in order from an object side, a positive first lens with a surface on the object side having a convex shape, a negative meniscus second lens with a concave surface facing an image side, a positive meniscus third lens with a convex surface facing the image side, and a positive or a negative fourth lens with both surface formed from aspheric surface and with a surface on the image side having concave shape in the vicinity of an optical axis. In such configuration, by setting the Abbe number of the first lens and the second lens to a preferable range, it becomes possible to obtain the correction effect of an axial chromatic aberration and a chromatic aberration of magnification, and by setting a ratio of a focal length of the second lens and the first lens and a ratio of a focal length between the third lens and the fourth lens, it becomes possible to secure telecentric properties and to correct the axial chromatic aberration and the chromatic aberration of magnification, while downsizing the overall lens system. Further, the imaging lens according to Japanese Patent Application Laid-Open No. 2007-264180 (Patent document 2) and Japanese Patent Application Laid-Open No. 2010-197665 (Patent document 3) propose practical lens solving the problems of the four-lens configuration, by utilizing the five-lens configuration.
According to Patent document 1, it becomes possible to obtain comparatively favorable aberration. However, in order to obtain sufficient resolution while corresponding to the small-sized, high-density imaging element as is explained above, a large aperture ratio such as F/2.4 is required. In Patent document 1, the total track length (TTL) is long, so that it is not suitable for downsizing. Further, it is difficult to realize favorable aberration correction while securing large aperture ratio. Patent document 2 and Patent document 3 propose practical lenses by solving the problems of the four-lens configuration, however, it is disadvantageous in cutting down costs since a large number of lenses must be used to configure the imaging lens. Further, since a large number of lenses sensible to manufacturing tolerance is used, it is disadvantageous also from the viewpoint of manufacturing cost. Also, since glass material is heavily used, it is disadvantageous from the viewpoint of reducing cost. Further, in Patent document 2 and Patent document 3, when plastic material is selected for decreasing cost, since the applicable lens material is extremely limited, it is difficult to achieve both correction of chromatic aberration and correction of other various aberrations.